[Simh] PDP-10/RP06 Disk Image Size

[Timothe Litt]

On 13-Apr-15 01:07, Rob Doyle wrote:
> I'm designing an RH11 disk controller for my KS10 FPGA. I'd like to
> use the SIMH data format for the media.
>
> I noticed that an RP06 disk image from SIMH is 315,187,200 bytes.
>
> Assuming an RP06 has 815 cylinders, 19 heads, 20 sectors (confirmed
> in SIMH code) - I calculate that disk image should be 317,132,800
> bytes.
>
> If use 810 cylinders (instead of 815), I get the size of the of the
> image file.
>
> SIMH doesn't pack data: there are 128 words per sector and each
> word occupies 64-bits or 8 bytes.  Therefore a sector is 1024 bytes.
>
> What's am I missing?

Maintenance mode.

The 5 highest cylinders of an RP06 are reserved for maintenance
operations -
providing a place for diagnostics to do writes and reads without
touching the
filesystem.  This is a software convention; the hardware doesn't have any
special support for this (though MSCP controllers do.)  The number of
maintenance cylinders was established for each drive type.

SimH won't have any reason to seek to them unless you run a timesharing
diagnostic that acesses
these cylinders, which you probably haven't.  So your host OS hasn't set
EOF out there.

There are special system calls that allow access under timesharing; e.g.
for TOPS-10, SU.SMN is set
in the SUSET. UUO to allow access to the maintenance cylinders.

From rhxkon.mac, the geometry tables:
|
||;BLOCKS PER UNIT||
||
||BLKPRU:    DEC    154280        ;(RP04/05) 406 CYLINDERS||
||    DEC    307800        ;(RP06) 810 CYLINDERS||
||    DEC    123150        ;(RM03) 821 CYLINDERS||
||    DEC    865504        ;(RP07) 629 CYL, 32 SURF, 43 SECT||
||
||;BLOCKS PER UNIT IN MAINTENANCE MODE||
||
||BLKPUM:    DEC    156180        ;(RP04/RP05) 411 CYLINDERS||
||    DEC    309700        ;(RP06) 815 CYLINDERS||
||    DEC    123450        ;(RM03) 823 CYLINDERS||
||    DEC    866880        ;(RP07) 630 CYLINDERS||
||
||;BLOCKS PER UNIT IN COMPATIBILITY MODE||
||
||BLKPUC:    DEC    171798        ;(RP04/RP05) 22*19*411||
||    DEC    340670        ;(RP06) 22*19*815||
||    DEC    131680        ;(RM03) 32*5*823||
||    0            ;(RP07) NO COMPATIBILITY MODE||
||
||;BLOCKS PER TRACK,,BLOCKS PER CYLINDER||
||
||BLKPTC:    XWD    ^D20,^D380    ;(RP04)||
||    XWD    ^D20,^D380    ;(RP06)||
||    XWD    ^D30,^D150    ;(RM03)||
||    XWD    ^D43,^D1376    ;(RP07)||
|
For TOPS-20, IIRC, you get to use DSKOP%.

Stand-alone diagnostics knew to restrict their operations to the
maintenance cylinders unless told by the operator that scribbling
was OK ("a scratch pack is mounted").

This communication may not represent my employer's views,
if any, on the matters discussed. 


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